This invention relates to the field of cathode ray tube deflection amplitude control, such as East-West waveform correction of horizontal deflection signals generated by a bus controlled integrated circuit.
The development of single integrated circuits containing both analog and digital television signal processing has greatly reduced receiver parts count, improved reliability and reduced manufacturing cost. Such integrated circuits frequently employ sync separation circuitry to lock a reference oscillator from which horizontal and vertical rate deflection signals are developed. To facilitate control of the IC functions with a minimum of circuit board potentiometers and to minimize the IC pin count, the integrated circuit may be controlled via a data bus. An example of a data bus system is the Thomson logic protocol which comprises three control lines, data, clock and enable respectively. The IC usually contains memory registers which store digital values that correspond to setup, alignment or user determined values for specific parameters. The stored digital data is converted to an analog value by a digital to analog converter. This analog value is coupled out of the IC to control the specific parameter in external circuitry.
To reduce the IC pin count, certain waveforms and control signals may output on common IC pins. For example, a horizontal pincushion correction waveform, namely a vertical rate parabola, may be output together with a horizontal width determining DC voltage. Thus a single IC pin is used for two circuit control functions. The selection of horizontal pincushion, and horizontal width control parameters is advantageous since both parameters may be controlled by a common deflection circuit configuration, for example a pulse width modulator coupled to a pincushion diode modulator. Thus the vertical rate parabola may be superimposed on a horizontal width determining DC voltage. However, this composite control signal requires DC coupling to the point of circuit control. Furthermore, control requirements may exist for certain deflection yoke/tube combinations which require control signal amplitudes which tend to exceed the output voltage swing capability of the multifunction IC. Hence constraints exist within the IC which limit the maximum amplitude ratio of the two control signals. Additional constraints exist within the IC in terms of the range of digital control, i.e. number of control data bits, and the consequential size requirements for control value memory.